The invention is directed to a process for the simple and economical production of U.sub.3 O.sub.8 powder having a crystal size of 40 to 90 .mu.m by calcining finely powdered uranium compounds of the oxidation stage +4 to +6 in an oxygen containing atmosphere at normal pressure (i.e., atmospheric pressure).
There have been efforts in operating material-test-reactors (MTR) of high degrees of uranium enrichment to switch to the use of fuels of lower degrees of enrichment of U-235. For the operation of MTR reactors having lower degrees of enrichment, however, the fuel density must be increased which becomes possible through the employment of oxidic uranium fuel materials of a specific crystal size, which preferably is between 40 and 90 .mu.m. Previously, for the production of this type of fuel, there were employed industrially expensive processes.
Thus, for example, there is known from German OS No. 3024634 and related Hackstein U.S. application Ser. No. 274,793 filed June 18, 1981, the entire disclosure of which is hereby incorporated by reference, to produce U.sub.3 O.sub.8 powder having a crystal size between 40 and 90 .mu.m by pressing a sinterable U.sub.3 O.sub.8 powder, sintering, breaking, and sieving the fine portion. Subsequently, the same process is repeated again with the large particles remaining behind on the sieve. In this procedure, the yield of particles whose size is between 40 and 90 .mu.m is relatively low, while the main amount of the U.sub.3 O.sub.8 powder is obtained as a non-utilizable fine portion.
Besides in the breaking, there is caused abrasion of the breaking tool, the abraded particles becoming an impurity in the U.sub.3 O.sub.8 powder, the quality of which thus can be reduced. As a further disadvantage of this process, mention is made of the development of dust in the breaking, which dust requires a corresponding closed process and leads to accumulation of secondary waste in the waste gas purification system.
Furthermore, it is known to start from an aqueous uranyl nitrate solution which is mixed with tetrahydrofurfuryl alcohol in order to produce a gel-like solution. This gel-like solution is run in from an oscillating nozzle tip having a fixed frequency. The spherical gel droplets pulling off from the nozzle tip run through a falling zone having an NH.sub.3 atmosphere and subsequently a column containing an aqueous NH.sub.3 solution, at whose bottom so-called nuclei collect as small yellow spheres, which contain the uranium in the form of ammonium diuranate. These nuclei are washed with an isopropanol/water mixture, subsequently dried in a vacuum, and then calcined. Particle growth occurs in the subsequent sintering step. Through sieving, the crystals of the desired size of 40 to 90 .mu.m can be obtained in a yield of about 70 to 80%. The fine particles and oversize particles must be rejected thereby and be reprocessed. This process, therefore, is very expensive in regard to apparatus and requires many process steps. Besides, secondary waste is produced in it in the form of aqueous, as well as isopropanol containing, ammonium nitrate solutions.
Besides, it is known to produce U.sub.3 O.sub.8 powder from uranium compounds of the stage of oxidation +4 to +6 by calcining in an oxygen containing atmosphere whereby as atmosphere there is used air, and the calcining takes place at a temperature below 1100.degree. C. A U.sub.3 O.sub.8 powder having a crystal size of 40 to 90 .mu.m, which besides should be as pore free as possible, cannot be produced in this manner.
Therefore, it was the problem of the present invention to find a process for the production of U.sub.3 O.sub.8 powder having a crystal size of 40 to 90 .mu.m by calcining of finely powdered uranium compounds of the oxidation stage +4 to +6 in an oxygen containing atmosphere at normal pressure which is economical, in which as little as possible secondary waste accumulates and which produces the most pore free crystals possible.